A new study suggests that human have been producing traceable amounts of atmospheric methane earlier than thought. The results will challenge global warming predictions, because what was assumed to be 'natural' levels of methane, have in fact been inflated by human activities since Roman times.

ADVERTISEMENT

An international team of researchers looked at carbon isotopes in methane trapped in air bubbles from Arctic ice cores, to reveal the different levels and concentrations of methane.

"With our research, we can now show that the emissions of methane have indeed been influenced by human activities for at least two thousand years," says lead researcher Dr Célia Sapart from Utrecht University, The Netherlands.

As a greenhouse gas, methane has a variety of both man-made and natural sources ranging from volcanoes and landfills to manure management, rice cultivation and field burnings.

Researchers were able to differentiate the methane sources because of their specific isotopic signatures, and therefore could be linked to their environmental drivers. The team combined their isotope data with information about past climate variation, population growth and changes in land use.

"It was clear that climate alone could not explain the changes we were seeing in the isotope data," explains Sapart.

Instead, the spikes of atmospheric methane matched a pattern of human events, including development from the Roman Empire and the Han Dynasty in China. These societies were marked with rapid population expansion leading to increased resource use. The researchers also could pick out spikes in a warm period known as the Medieval Climate Anomaly and a cold period known as the Little Ice Age.

Revealing that human-induced methane was emitted prior to the industrial revolution where fossil fuel consumption really started to take off, changes the way we should asses global warming cycles.

These new findings will allow us to understand how natural methane emissions will respond to future climate conditions.

Sapart states: "If we hope to predict how methane levels are likely to vary in the future we must understand how natural methane sources have behaved in the past. But our findings show that man has influenced atmospheric methane for much longer than we thought, so we have to look further back in time to be sure we are looking at the behaviour of natural methane emissions."